ARM is simply the most common and practically used embedded microprocessor all over the world .. with a nose-to-nose other processor called MIPS (like Intel and AMD) .. both processors are used in embedded systems .. like mobile phones .. PDA's .. and everything that has software adjacent to hardware .. away from Personal Computers ..
ARM is a RISC processor .. which means an architecture different from most Personal Computer Microprocessors (they are CISC) ..
ARM has many families .. you can check for them in www.arm.com